1. Field of the Invention
The present invention relates generally to optical waveguide fibers, and more particularly to an optical fiber exhibiting single polarization properties, and polarization maintaining properties.
2. Technical Background
Single polarization optical fibers are useful for ultra-high speed transmission systems or for use as a coupler fiber for use with, and connection to, optical components (lasers, EDFAs, optical instruments, interferometric sensors, gyroscopes, etc.). The polarization characteristic (single polarization) propagates one, and only one, of two orthogonally polarized polarizations within a single polarization band while suppressing the other polarization by dramatically increasing its transmission loss.
Polarization retaining fibers (sometimes referred to as a polarization maintaining fibers) can maintain the input polarizations on two generally-orthogonal axes. A common polarization maintaining fiber includes stress birefringence members and includes, as shown in FIG. 1, a central core 10 surrounded by an inner cladding region 11. Core 10 and cladding region 11 are formed of conventional materials employed in the formation of optical waveguides. The refractive index of the core material is greater than that of the cladding material. By way of example only, core 10 may consist of silica containing one or more dopants which increase the refractive index thereof, such as germania. Cladding region 11 may comprise pure silica, silica containing a lesser amount of dopant than core 10, or silica containing one or more down dopants, at least one of which is an oxide of an element such as boron or fluorine which lowers the refractive index of silica. Polarization maintaining fiber has high birefringence (Δn), which is defined by the difference of refractive index between the two polarizations of the fundamental mode. The light belonging to each state of polarization travels at a slightly difference velocity in the fiber at a given wavelength λ. The birefringence determines the distance to accumulate one wavelength of phase delay between the two polarization modes, which is referred to as beatlength LB. The fiber beatlength LB is related to the fiber birefringence (Δn) in a simple equation,LB=λ/Δn.The strength of the polarization maintaining capability is often gauged by the fiber birefringence or beatlength. The shorter the beatlength, the higher the birefringence and therefore the better the state of polarization is maintained. It can be found from the above equation that the beatlength is linearly proportional to the wavelength, which means that for a fixed birefringence Δn, the beatlength increases with wavelength.
In FIG. 1, diametrically opposed relative to core 10, are two stress-inducing regions 12 formed of a glass material having a Thermal Coefficient of Expansion (TCE) different from that of cladding material 11. When such a fiber is drawn, the longitudinally-extending regions 12 and the cladding regions disposed orthogonally thereto will shrink different amounts whereby regions 12 will be put into a state of tension or compression strain. Strain induced birefringence (otherwise referred to a stress-induced birefringence) is imparted in the fiber and thereby reduces coupling between the two orthogonally polarized fundamental modes. Surrounding regions 12 is an outer cladding region 13, the refractive index of which is preferably equal to or less than that of inner cladding region 11. Region 13 may consist, for example, of any of the materials specified above for use as cladding region 11. It should be recognized that such fibers including these stress-inducing regions 12 do not provide single polarization properties.
Slight improvement in the polarization performance of single mode optical fibers has been achieved by elongating or distorting the fiber core geometry, as a means of decoupling the differently polarized waves. Examples of such optical fiber waveguides with elongated cores are disclosed in U.S. Pat. Nos. 4,184,859, 4,274,854 and 4,307,938. Prior Art FIG. 2 illustrates a waveguide 1 having a core 4 having refractive index, n1, a cladding 5 having a refractive index, n2, wherein the elongated core 4 has a major axis, a, and a minor axis, b. However, the noncircular geometry alone is, generally, not sufficient to provide the desired single polarization properties.
It has, therefore, been an area of ongoing development to obtain an optical fiber that will provide single polarization performance, and which is also easily manufacturable.